Approximately 40 million people are infected with HIV world-wide and 10% of these individuals will die each year from AIDS. In addition, the annual number of new infections is estimated to be 5 million and rising. The cost of treating this disease is enormous, and varies from $2,500 per patient in Brazil to over $15,000 per patient per year in developed countries. Cost of prevention is estimated at more than $120 billion over the next 10 years, although the long-term benefit from prevention would dramatically reduce future costs for treatment and care. The bulk of the cost of current treatment is for anti-retroviral drugs, which are remarkably effective but often lead to resistance. Furthermore, long-term control of the infection, most likely by management as a low-grade, chronic disease, increases the cost burden beyond that which can be afforded in low- and middle-income countries.
HIV-1 enters into cells by first attaching to one or more receptors on a cell, thereby inducing conformational and/or structural changes that allow insertion of the viral genome into the cell. Once inside the cell, the viral RNA genome is transcribed into DNA, integrated into the host genome, and then free to replicate. The primary therapies against HIV infections are anti-retroviral drugs that inhibit viral replication after entry into the cell. The most commonly used are nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors that block enzymatic processing of viral products. These drugs effectively inhibit replication of the virus inside an infected cell and reduce viral load in the blood to undetectable levels.
Another therapeutic approach uses fusion inhibitors, including proteins (e.g. monoclonal antibodies), peptides and small molecule agents (e.g. drugs), some of which act on the outside of the cell to prevent HIV from fusing with and infecting it. If HIV cannot penetrate the host cell membrane and infect the cell, then HIV cannot replicate. Fusion inhibitors effectively block infection by HIV-1 and significantly reduce the systemic viral load. Vaccines that elicit antibodies that inhibit such fusion are of interest in this regard, and several pharmaceutical companies are working to achieve this goal.
Combinations of small molecular weight drugs, however, achieve undetectable levels of HIV virus in only about 50 to 60% of treated patients. In addition, the development of treatments that involve antibodies is generally costly and requires considerable medical infrastructure. Furthermore, although the development of prophylactic treatments such as vaccines is an important effort, particularly for susceptible target populations, this approach has thus far been unsuccessful. Protocols must be developed for those already infected.
In contrast to therapeutic approaches aimed at prevention or control of the disease by directly inhibiting a step in the viral replication cycle, as described above, reactivation of patients' immune system is an alternative therapy that holds promise for restoring health and productivity to an infected patient in a practical, cost-effective manner. As a result, an intense interest in immunotherapy, as indicated by the development of cytokine treatments for example, is leading to products that can stimulate or inhibit the immune system. One developmental cytokine/immunomodulator project for the treatment of HIV/AIDS has, for example, identified two key peptides derived from Thymus Nuclear Protein (TNP) technology (Viral Genetics, Inc., Azusa, Calif.), These peptides occur naturally in a variety of mammals, including humans.
The role of cytokines in the inhibition of HIV infectivity, particularly interleukin-16 (IL-16), interleukin-8 (IL-8) and RANTES (Regulated upon Activation, Normal T-cell Expressed, and Secreted; also known as CCL5), is very important. HIV-1 enters cells by first binding to two key molecular components on the cell surface, the protein CD4 and co-receptors CCR5 or CXCR4. CD4(−) cells are therefore insensitive to HIV, and genetic inactivation of CCR5 correlates strongly with resistance to HIV-1 infection. Cytokines such as IL-16, IL-8 and RANTES, which have overlapping and complementary functions, can act to attenuate viral infection by competing with viral binding and by interfering with viral entry into cells by down-regulating the receptors required for entry. Other cytokines such as interferons (e.g. IF-.alpha. and IF-.gamma.) act to reduce viral load by stimulating antibody-mediated phagocytosis.
Interleukins (IL's) and interferons (IF's) are potent cellular stimulants that are released from a variety of cells in response to insult or injury. Consequently, these proteins have attracted intense interest as therapeutic agents. However, similar to general stimulants such as lipopolysaccharide (LPS), IL's and IF's induce release of inflammatory cytokines and thus, when given at higher than normal concentrations during therapy, have substantial adverse effects resulting from inflammation which can be life-threatening and may require inpatient treatment facilities. Similarly, levels of TNF-α, IL-1β and IL-6 are directly correlated with the probability of death in humans. Moreover, production of recombinant IL's and IF's and their application are very costly, and even lower-dosage immunostimulant treatments developed for out-patient use have lower success rates and are not suitable in some situations such as, for example, to extend remission from cancer therapy or control a disease such as HIV at a chronic level.
In general, a stimulant of IL-8 and IL-16 release appears to be particularly suited for a role in enhancing host defense. Selective release of IL-8 by monocytes is possible without the release of inflammatory cytokines such as IL-1β and IL-6. However, a potentially adverse effect of IL-8 production is the enhanced recruitment of neutrophils to inflamed endothelial cells and subsequent release of cytotoxic factors which cause cell/tissue damage, in addition to the continued production of IL-8 by adjacent (non-inflamed) endothelial cells. The consequence is a vicious cycle of recruitment of neutrophils in response to IL-8, damage to tissues, and more production of IL-8, although higher concentrations of IL-8 can be beneficial when they lead to internalization of receptors and de-sensitization of the cells. Therefore, exogenous therapeutic agents such as large, intact cytokine molecules are not well suited for general therapeutic use.
Information relevant to attempts to address one or more of these problems can be found in the following references: U.S. Patent Publication No. 2007/0003542; U.S. Patent Publication No. 2006/0269519; U.S. Patent Publication No. 2004/0248192; P. W. Latham, 1999; Fatkenheuer et al., 2005; Stover et al., 2006; Cohen, 2007; GlaxoSmithKline, 2005a and GlaxoSmithKline, 2005b. However, each one of these references suffers from one or more of the following disadvantages:
1. the size or composition of the agent provides significant challenges to cost-effective synthesis and purification;
2. the agent is specific for particular pathogen and/or cell type, rendering them unsuitable for general therapeutic use;
3. treatment with the agent induces clinically deleterious side effects that can be life-threatening, such as inflammation or hepatotoxicity, and require inpatient treatment facilities;
4. termination of treatment is followed soon thereafter by an increased systemic viral load;
5. long term exposure to agent often leads to treatment-resistant pathogens;
6. lower-dosage treatments developed for out-patient use have lower success rates and are not suitable in some situations;
7. treatment is ineffective, impractical, or cost-prohibitive for a large proportion of patients;
8. development of therapeutic antibodies require considerable medical infrastructure;
9. treatment such as vaccines may be appropriate to prevent infection but not to treat those already infected;
10. no beneficial synergy between the immunogenic response induced and the effects of other endogenous immunoregulators;
11. agent inhibits the release of inhibitory cytokines that suppress release of beneficial cytokines, an indirect treatment; and
12. agent acts to restore baseline cytokine levels to balance responses of the immune system rather than promoting activation of phagocytes.
Many of these therapeutic protocols also become ineffective with time because mutation of the pathogen allows it to escape the treatment. Moreover, any immunosuppression that accompanies the disease attenuates the ability of the innate immune system to respond to antigenic changes and thereby keep the infection under control. However, even though the virus may mutate at one or a few sites and thereby escape the neutralizing activity of antibodies, endogenously produced antibodies are usually polyclonal and may still bind the virus.
The immune system in individuals infected with a pathogenic agent such as HIV initiates a defense response by production of antibodies. The presence of anti-HIV antibodies is often used as a diagnostic test for infection. During the course of the disease, the antibody level remains high whereas the ability to maintain a minimal viral load gradually weakens as the population of CD4+ T cells declines. The cellular components of the innate immune response then become absent or quiescent. When the immune defense mechanisms reach a sufficiently low level, viral replication is not held in check and rapidly leads to a final stage of the disease, designated AIDS. However, even at this late stage, patients can be rescued from death by aggressive anti-retroviral therapy. Therefore, an agent that reactivates cells of the immune system, in particular phagocytes, will likely also restore an immune defense against progression of the disease.
In light of the available treatments for infections such as HIV induced AIDS, there are large numbers of people worldwide that need alternative, practical, cost-effective, non-specific therapies that directly bolster a patient's immune system during the course of the disease without causing deleterious side effects. Ideally, such therapies should also be effective against other types of pathogens.
Therapeutic agents that activate/reactive the immune system show particular promise in this regard, including cytokines and immunomodulators, although therapies based on exogenous agents such as large, intact cytokine molecules are not generally well suited for therapeutic use. Peptides, however, are often much more suitable therapeutic agents than large polypeptides or proteins. Peptides can, for example, be designed to induce one or more particular desired effects in vitro or in vivo, often without concomitantly inducing deleterious effects, and can usually be synthesized in a cost effective manner.